Benthic Foraminifera in Siliceous Black Ore from the Ezuri Kuroko Deposit, Hokuroku District, Japan

Abstract: Abundant benthic foraminifera have been identified in thin sections of the siliceous black ore in the Ezuri Kuroko deposit, Hokuroku, Japan. By treating samples with conventional hydrofluoric acid digestion techniques, sponge spicules and radiolaria have also been recognized in the residue. Under microscopic observations, 94 individual foraminiferal specimens have been detected. However, as it is difficult to identify species or genera by means of microscope observations alone, only a small number of genera have been identified based on morphology. The foraminiferal assemblage is composed predominantly of agglutinated species (83%) with subordinate calcareous species (17%), and is assigned to the Cyclammina Assemblage based on the preponderance of Cyclammina (57%). The foraminifera are generally well preserved within micro‐crystalline to cryptocrystalline quartz, and exhibit no obvious features related to compaction or secondary deformation. Textural observations suggest that the siliceous component of this rock was not derived from an allochthonous block but instead constitutes autochthonous proto‐Kuroko sediment. The Cyclammina Assemblage in the ore is different from recently described foraminiferal assemblages in the vicinity of present deep‐sea hydrothermal vents, but is identical to those found in black shales of the Onnagawa to Funakawa stages in the Green Tuff region. The proto‐Kuroko sediment is assumed to have been deposited in an oxygen‐deficient environment within a closed, deep‐seated basin. The existence of siliceous microfossils suggests that the silica in the siliceous ore did not originate from silica sinter deposits produced by submarine hydrothermal activities, but from a biogenic siliceous ooze, probably composed of diatoms. Sulfide mineralization in the interstices of some of the microfossils is inferred to relate primarily to bacterial sulfate reduction associated with the decomposition of organic matter. The later sulfide mineralization associated with larger crystals (which contain fluid inclusions with homogenization temperatures of approximately 250C) cuts across the siliceous masses and foraminiferal septa, and may have been formed after consolidation of the siliceous ooze, accompanying the formation of acidic intrusive rocks during the late Onnagawa stage.